Vacuum deposition method is known as one of the procedures for forming a thin film, and this vacuum deposition method consists of steps of placing raw material(s) for deposition in a crucible, heating the crucible to a temperature above the vaporization point of the deposition material in the vacuum deposition apparatus to thereby create the vapor of the deposition material, and depositing the substance on a substrate to thereby form a thin layer. As a method for heating the crucible, resistance heating, electron beam heating, high frequency induction heating or the like have been used, but among them the resistance heating is well known wherein heating element consisting of tantalum wire or tungsten wire is held outside the crucible by an electrically insulating device in a manner such that the heating element is wound about the crucible's outer periphery (IP Publication 1).
However, in the case of the resistance heating method of IP Publication 1, the heating element has no cladding layer so that the conductor is exposed when in use and thus there is a fear of discharge of electricity and short circuit. Also, as the heating element is heated to high temperatures, the heating element is apt to undergo degradation through embrittlement and dissipation with a possible consequence of wire breakage.
On the other hand, IP Publication 2 teaches about a ceramic heater, according to which a supporting base body made of pyrolytic boron nitride (PBN) by chemical vapor deposition method is coated with a electrically conductive thin layer of pyrolytic graphite (PG) by means of chemical vapor deposition method, and then by machining this PG layer a desirously patterned heating element is obtained, and this heating element is paved with a pyrolytic boron nitride insulating layer by means of chemical vapor deposition method, and hence the ceramic heater (PG/PBN heater) is made.
The ceramic heater of IP Publication 2 has its heating element paved with insulating layer so that there is no fear of discharge of electricity and short circuit, and also since the supporting base body, the heating element and insulating layer are all made by chemical vapor deposition method, the ceramic heater has a relatively high purity and scarcely releases impurities.
However, this ceramic heater is a flat plate-type heater used for uniformly heating a base plate to make a semiconductor wafer or a thin film so that it is not possible to use this ceramic heater as it is for the application of heating a raw material-holding crucible in a vacuum deposition apparatus. Also, even when it is used for heating a raw material-holding crucible to a temperature of 1000 degrees centigrade or higher, the heater pattern of the ceramic heater is such that an electric current passing a folded-back section of the heater pattern tends to converge toward the inner corner so that the vicinity of the inner periphery of the folded-back section is locally heated to an extra higher temperature than the outer periphery thereof, and consequently a problem exists that for a long term use the heater cannot be expected to perform stably and durably.
IP Publication 3 describes a ceramic heater wherein the folded-back section of the heater pattern is divided into a number of lanes that run along the direction of the electricity flow passage in order to solve the said problem of uneven electricity flow had by the conventional flat plate-type heater. However, the thus improved ceramic heater is designed to uniformly heat a flat plate body, so that it is not suitably used in heating a three-dimensional raw material-holding crucible to a heightened temperature. Also, there is no teaching in IP Publication 3 as to whether or not the improved ceramic heater can withstand a long term use wherein the raw material-holding crucible is heated to a temperature of 1000 degrees centigrade or higher.